1. Field of the Invention
The present invention relates to a device utilizing a jet of liquid and more particularly a recording or printing device in which selectively charged ink droplets are flung towards a suitable record carrier such as a sheet of paper.
2. Description of the Prior Art
Printing devices are known, in which conductive ink under pressure is fed to a small size nozzle and emerges from this nozzle in the form of a jet of ink which, because of instabilities due to the surface tensions exerted at its surface, is divided into a series of droplets. By means of a transducer which causes the nozzle to be placed in oscillation in a direction parallel to the direction of the jet, droplets are obtained of which the diameter and spacing are uniform. These droplets are flung towards a record carrier or printing medium, and are deflected during their travel so that upon impinging on this surface, they form sets of dots which represent letters, numerals or in more generalized manner, some form of symbols.
In particular printing devices of this type, for example are described and illustrated in French Pat. No. 2,204,149. The ink droplets are generated by means of a single nozzle which is moved with respect to the record carrier in a continuous manner along a direction parallel to the lines of the characters which are to be printed. In these devices, the impression of each character is obtained by deflecting the droplets which are sprayed by this nozzle in a direction at right angles to that of the relative movement of the nozzle, this deflection being performed in repetitive manner during this relative movement. To this end, a charging electrode situated close to the point at which the ink jet issuing from the nozzle is divided into droplets, selectively applies an electrostatic charge to each droplet upon being formed, the magnitude of this charge being a direct function of the voltage of the control signal applied to the charging electrode. Only the droplets needed for printing characters are charged in this manner, whereas those not required for printing are not charged. All these droplets then pass through a constant electrical field generated by two deflecting plates between which is maintained a comparatively high difference in potential, this field being aligned in a difference at right angles to that of the discharge from the nozzle. This has the result that the charged droplets which are needed for printing the characters, are deflected in the direction of this field, and reach the record carrier to form the required character on the same, the magnitude of this deflection being a function of the charge borne by each of the droplets. By contrast, the uncharged droplets which are not needed for printing, are not deflected and are then collected by a trough towards which they are directed. However, although the droplets generated by the nozzle are flung at a speed V.sub.L in a direction at right angles to the surface of the record carrier, the nozzle imparts a complementary speed V.sub.G to each of these droplets as a result of its movement with respect to this carrier. The speed V.sub.G acting in alignment with the direction of movement of the nozzle, that is to say at right angles to the direction of discharge of the droplets. By reason of the combined action of these two speeds V.sub.L and V.sub.G, each droplet follows a path contained in a plane which is not at right angles to the surface of the record carrier. In view of this fact, the droplets which are dispatched towards the record carrier reach the latter at an angle of inclination such that they are spread out upon striking, each forming an elongated spot of substantially elliptical shape. The letters formed by the totality of these spots then have a blurred appearance which greatly impairs their legibility. This phenomenon, which is appreciable primarily in the direction of the movement of the nozzle, is the more intensive as the droplets are coarser and the higher the ratio is between the speed V.sub.G of relative displacement of the nozzle and the speed V.sub.L at which the droplets are propelled. In the prior printing devices, this ratio was comparatively low, so that the droplets underwent practically no spreading upon striking the record carrier. However, in the printing devices produced at present, it has been attempted to raise the speed V.sub.G of movement of the nozzle in order to increase the rate of printing characters, so that the phenomenon of spreading of the droplets become particularly troublesome.
The present invention seeks to eliminate this disadvantage and provides a recording device utilizing a jet of liquid, in which the spots formed when the droplets of liquid strike the record carrier remain practically circular.